From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, T_DKIMWL_WL_HIGH,USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5D276C282CE for ; Tue, 4 Jun 2019 09:26:50 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 3318824635 for ; Tue, 4 Jun 2019 09:26:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="QrN4OS8R" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3318824635 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=jgUg5N1QuP9WVCtgUgcfUjM8NhLImrry/FDmLEwyTjM=; b=QrN4OS8R5SunZ0 PR6jv2fYFGOQQFW1LSwPzaMCiGZiOVSZ7h16PkBO7zWoCEG8gKii8mADGb0b7+4oGnxiUbZicZjx7 iVfWsHFqig/yv9e6c5oww2x51XA1HrcjprGl6EEGKw61b95+VCMTPQH56wDuFOQkSjOhA3HPj2quz uRw3nF0pPygMlqfMIhvFtmCwg4YVFqdwkyq8flnOsxMo8a/dsiohWenIalPHSgbpi7VgWpbnjpYRf dev4rbO7HUF+f/PromQw6V/pVV9L/8nz4HJvgkC2jAHtbLYcPRRLveW10P0Hw7RyLSyzs/fANAdQt Chaptl7IvuSQsCOVI3bQ==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1hY5iS-0004WU-1f; Tue, 04 Jun 2019 09:26:48 +0000 Received: from foss.arm.com ([217.140.101.70]) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1hY5iO-0004Vp-Gh for linux-arm-kernel@lists.infradead.org; Tue, 04 Jun 2019 09:26:45 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4D819A78; Tue, 4 Jun 2019 02:26:43 -0700 (PDT) Received: from e103592.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C62673F246; Tue, 4 Jun 2019 02:26:41 -0700 (PDT) Date: Tue, 4 Jun 2019 10:26:39 +0100 From: Dave Martin To: Viresh Kumar Subject: Re: [PATCH] KVM: arm64: Drop 'const' from argument of vq_present() Message-ID: <20190604092639.GS28398@e103592.cambridge.arm.com> References: <699121e5c938c6f4b7b14a7e2648fa15af590a4a.1559623368.git.viresh.kumar@linaro.org> <20190604084349.prnnvjvjaeuhsmgs@mbp> <20190604085545.hsmxfqkpt2cbrhtw@vireshk-i7> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20190604085545.hsmxfqkpt2cbrhtw@vireshk-i7> User-Agent: Mutt/1.5.23 (2014-03-12) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190604_022644_559881_E4E65DC3 X-CRM114-Status: GOOD ( 18.32 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Marc Zyngier , Catalin Marinas , Will Deacon , linux-kernel@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jun 04, 2019 at 02:25:45PM +0530, Viresh Kumar wrote: > On 04-06-19, 09:43, Catalin Marinas wrote: > > On Tue, Jun 04, 2019 at 10:13:19AM +0530, Viresh Kumar wrote: > > > We currently get following compilation warning: > > > > > > arch/arm64/kvm/guest.c: In function 'set_sve_vls': > > > arch/arm64/kvm/guest.c:262:18: warning: passing argument 1 of 'vq_present' from incompatible pointer type > > > arch/arm64/kvm/guest.c:212:13: note: expected 'const u64 (* const)[8]' but argument is of type 'u64 (*)[8]' > > > > Since the vq_present() function does not modify the vqs array, I don't > > understand why this warning. Compiler bug? > > Probably yes. Also marking array argument to functions as const is a > right thing to do, to declare that the function wouldn't change the > array values. > > I tried a recent toolchain and this doesn't happen anymore. > > Sorry for the noise. Sparse is already warning about this, but I had dismissed it as a false positive. I think this is an instance of disallowing implicit conversions of the form T ** -> T const ** because this allows a const pointer to be silently de-consted, e.g.: static const T bar; void foo(T const **p) { *p = &bar; } T *baz(void) { T *q; foo(&q); return q; } I _suspect_ that what's going on here is that the compiler is eliminating a level of indirection during inlining (i.e. converting pass-by-reference to direct access, which is precisely what I wanted to happen). This removes the potentially invalid behaviour as a side-effect. This relies on the compiler optimising / analysing the code aggressively enough though. So, I don't have a problem with dropping the extra extra const, e.g.: static bool vq_present( u64 (*const vqs)[KVM_ARM64_SVE_VLS_WORDS], unsigned int vq) Since this function is static and only used very locally, I don't see a big risk: the only reason for the extra const was to check that vq_present() doesn't modify vqs when it shouldn't. But it's a trivial function, and the intent is pretty clear without the extra type modifier. I'm in two minds about whether this is worth fixing, but if you want to post a patch to remove the extra const (or convert vq_present() to a macro), I'll take a look at it. Cheers ---Dave _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel